git config --get-all not supported

[masukomi]

Maybe I'm missing something obvious but it appears that Config.fetch does not support the ability to pull back an array of values like you would get with the --get-all option to git config and there is no other method that supports it.

Suggestion: add a fetch_all method that always returns an array (potentially empty if there are no matching keys)

[bkeepers]

Grit is no longer maintained. See #183 and check out libgit2/rugged.